If your frequent flier miles are languishing unused in your account because you keep coming up against blackout dates whenever you try to use them, then perhaps it’s time to switch airlines, or maybe even rethink your booking tactics.

A survey by IdeaWorksCompany looked at 25 of the biggest frequent flier programs to figure out which carriers were best when it came to redeeming frequent flier points. The study checked each of the airline’s primary routes to see what level of availability there was for travelers looking to book a flight using their miles.The results? Coming out on top were Southwest Airlines, Air Berlin and GOL, which all showed reward seat availability 100 percent of the time.

Among the big players, United Airlines took out eighth place on the list with 80 percent availability. Other large airlines did worse, with American Airlines making reward seats available about 49 percent of the time, while travelers trying to redeem miles on Delta or US Airways would have been successful only 36 percent of the time.

So why did budget airlines fare so well in the study? According to the company behind the survey, low cost airlines focus on offering short- and medium-haul flights that operate multiple times a day – meaning there are simply more seats to go around. The same principle applies if you’re trying to book a short-haul flight on a major airline, with reward availability hovering around 85 percent for flights under 2,500 miles. It’s the long-haul intercontinental flights with lower seat density that drag the big carriers down the rankings.

Some experts also believe the reason budget airlines keep topping the survey is that fewer business travelers (who tend to dominate frequent flier program enrollment) use them – meaning that when it comes time to claim your reward miles, you don’t have to compete with as many people for your seat.